The overall goals are to define the structure and function of vestibulospinal pathways in the generation and control of head stabilization and movement in primate. The significance of head stabilization is seen by our ability to change gaze orientation and maintain binocular fixation during most activities, and our ability to detect self-motion and adaptively react to externally-imposed head/body perturbations. Disorders of the vestibular system and cervical spine affect head stability, and can dramatically alter the patient's willingness to induce self-motion and can lead to trepidation of normal activities. Despite the importance, wide gaps remain in our understanding of basic vestibulospinal mechanisms. We propose that the vestibulospinal pathways represent distinct channels through which the labyrinth controls or biases brainstem and spinal sensory and motor circuits; the recruitment of these channels depends upon the nature of the head motion or perturbation, the instantaneous movement and orbital position of the eyes, and the orientation of the head/neck on trunk; and both channels are under voluntary gaze control. The proposed studies will directly test these hypotheses. Cells of the lateral vestibulospinal tract (LVST) are the principal focus of the proposed studies. There are three essential questions: where do they project, who do they target, and what signals do they carry. (l) In the esthetized, paralyzed animal LVST axons supplying the upper cervical segments and those innervating also the cervico- thoracic junction will be identified using electrical and natural canal/otolith stimulation and intracellularly labeled with biocytin to specify their brainstem and intraspinal morphology. (2) Using intracellular techniques, vestibular-target neurons that likely participate in vestibular-neck reflexes, e.g. specific neck/back otoneurons, propriospinal, and precerebellar neurons, will be identified, their synaptic input from vestibular and proprioceptive receptors assessed, and their soma-dendritic and axon morphology specified by biocytin labeling. (3) In the alert, trained monkey, chronic unit and eye movement (including torsion and vergence) recording and electrical stimulation techniques will be used to identify C1- and C7- projecting LVST cells, and characterize their discharge to imposed angular and linear head acceleration and whole body tilt and to voluntary ocular tracking and near viewing fixation; the synaptic contribution of irregular canal and otolith afferents to the cell's acceleration response also will be assessed using an electrophysiological paradigm.